The wireless power transfer method is roughly divided into an electromagnetic induction type, a radio wave reception type, and a magnetic field resonance type.
The electromagnetic induction type of wireless power transfer method has already been put to practical use to charge a battery of an electric toothbrush or an electric shaver. This method is efficient but easily affected by misalignment between a power transfer module and a power reception module. Therefore, they are arranged so as to almost closely contact with each other. For example, they are used in such a manner that a special power reception module is fitted firmly into a special power transfer module.
The radio wave reception type of wireless power transfer method is such that electrical power is transferred in the form of electromagnetic waves and the power reception side receives the electromagnetic waves and converts them into electrical power. The radio wave reception type has been actually used in a penguin house and the like in Pittsburg Zoological Gardens. This method is free from the restriction of causing the power transfer module and the power reception module to almost closely contact with each other in the electromagnetic induction type of wireless power transfer method. However, the energy use efficiency of this method is as low as less than one percent when, for example, a dipole antenna is used at 900 MHz at a distance of two meters. Therefore, the power transfer module and power reception module have to be isolated sufficiently. In addition, what this method is actually applied to is limited to an application that features very low power consumption.
The magnetic field resonance type of wireless power transfer method was demonstrated by researchers of Massachusetts Institute of Technology in 2007 as a method for overcoming the problems of the above methods to some degree. They succeeded in supplying electrical power to a sixty-watt light bulb at a distance of two meters at an energy use efficiency of about fifty percent and turning on the bulb. In this method, each of the power transfer module and power reception module includes a resonator that resonates at a predetermined frequency, thereby causing an alternating-current magnetic field at the frequency to concentrate efficiently near the power transfer and reception modules. As a result, in the future, this method will find applications in supplying a large amount of power, such as supplying power to trams. Efficient transfer is conditional on that the Q-factor of the resonator is high.
Some sort of communication function between the power transfer module and the power reception module for authentication, declaration of power requirements, or the like is essential to put the wireless power transfer system using the aforementioned techniques to practical use.
The magnetic field resonance type of wireless power transfer method has a great amount of potential in terms of electrical power transfer distance, efficiency, transferred power amount, and the like. Therefore, the magnetic field resonance type of wireless power transfer which achieves power transfer and data communication efficiently between the power transfer module and power reception module with the minimum configuration has been desired.